If chick
embryos develop to the pipping stage, or at first shell cracking
at hatching, they are normally healthy enough to hatch unless
some incubator adjustment prevents it from happening. The
problem is usually caused by either

1) poor
ventilation

2)
improper humidity.

The air exchange requirement within an incubator is greatest during the
last day of incubation. The chick embryo's oxygen requirement
continually increases during development and especially when
breathing using the respiratory system just before hatching. The
vent openings are frequently restricted at this time in an
attempt to boost incubator humidity. Instead of helping the
chick hatch, the chick is suffocated from lack of ventilation.
Never decrease ventilation openings at hatching in an attempt to
increase humidity. Increase humidity by other methods. If any
vent adjustments are made, they should be opened more.

Another
reason for mortality during hatching is improper humidity
adjustment. The deaths can be produced from too much humidity
during the entire incubation period or from too little humidity
during the hatching period.

The
desired egg weight loss during incubation caused by water
evaporation is about 12 percent. If humidity during incubation
is kept too high, adequate water evaporation from the egg is
prevented. The chick can drown in the water remaining in the
shell at hatching. A dried coating around the chick's nostrils
and beak indicates that drowning was likely. Attention to
maintaining proper incubation humidity during incubation will
reduce the potential for this problem at hatching time.

If the
humidity is allowed to decrease after the chick pips the shell,
the membranes within the shell can dry-out and stick to the
chick. This prevents the chick from turning inside the shell and
stops the hatching process. The chick eventually dies. If the
membranes around the shell opening appear dried and shrunken,
the cause is probably low humidity during hatching. This
condition can occur quickly (within 1 or 2 minutes) when the
incubator is opened to remove or assist other chicks that are
hatching. When hatching begins and proper incubator conditions
are attained, the incubator should never be opened until after
all chicks are hatched and ready for placement in the brooder.